1. Field of the Invention
The present invention relates to a converter configured to convert a voltage, a switching power supply including the converter, and an image forming apparatus including the switching power supply.
2. Description of the Related Art
FIG. 9 illustrates an example of a block diagram of a device that includes a typical switching power supply. In FIG. 9, an AC/DC converter 100 transforms an alternating-current voltage from a commercial power supply 1 to a direct-current voltage Vout1. The voltage Vout1 is supplied to an actuator 101 such as a motor. The voltage Vout1 is also supplied to a DC/DC converter 102. The DC/DC converter 102 transforms the voltage Vout1 to a direct-current voltage Vout2. The voltage Vout2 is supplied to a control unit 103 configured to control the device.
Generally, the power supply voltage Vout1 for the actuator 101 is set higher than the power supply voltage Vout2 for the control unit 103. For example, when the voltage Vout1 is DC 24.0 V (Vout1=DC 24.0 V), the voltage Vout2 is set to DC 3.30 V (Vout2=DC 3.30 V) or when the voltage Vout1 is DC 12.0 V (Vout1=DC 12.0 V), the voltage Vout2 is set to DC 1.80 V (Vout2=DC 1.80 V) in general. In the following explanation, an example in which the voltage Vout1 is DC 24.0 V (Vout1=DC 24.0 V) and the voltage Vout2 is DC 3.30 V (Vout2=DC 3.30 V) will be described. Here, “AC” represents an alternating-current voltage and “DC” represents a direct-current voltage.
As a typical switching power supply, a configuration of power supply including an AC/DC converter and a DC/DC converter is discussed in Japanese Patent Application Laid-Open No. 2003-266878.
In these days, further reduction of power consumption of an electronic device in the standby state has been strongly desired. In the device including a switching power supply discussed in the aforementioned Japanese Patent Application Laid-Open No. 2003-266878, a normal operation mode and a power saving mode for reducing power consumption are provided. In the power saving mode, the operating state of the switching power supply is changed to reduce standby power consumption. An operation of the switching power supply will be described below with reference to FIGS. 9 and 10.
FIG. 10 is a circuit diagram of a switching power supply including an AC/DC converter 100 for converting an alternating-current voltage from a commercial power supply into a direct-current voltage, and a DC/DC converter 102 for converting the direct-current voltage from the AC/DC converter 100 into a different direct-current voltage. Here, in the switching power supply, the AC/DC converter serves as a first converter and the DC/DC converter serves as a second converter. The output voltage of the AC/DC converter is a first voltage and the output voltage of the DC/DC converter is a second voltage. In this example, the switching power supply is discussed as a switching power supply that outputs a first voltage of 24.0 V and a second voltage of 3.30 V. Next, a basic operation of the switching power supply will be described.
First, the AC/DC converter 100 serving as a first converter will be described. The alternating-current voltage of the commercial power supply 1 is rectified and smoothed by a bridge diode 2 and a primary smoothing capacitor 3 and output as a direct-current voltage. The direct-current voltage is supplied to a field-effect transistor (FET) 9 serving as a switching element via a primary winding 10p of a transformer. To a gate terminal of the FET 9, a pulse-width modulation (PWM) circuit including a comparator 5, a triangular wave generator 4, a constant voltage source 6, a resistor 7, and a photocoupler 8p is connected. The FET 9 thus performs PWM switching based on error information of the output voltage fed back to the photocoupler 8p. When PWM switching is performed, this introduces a pulse voltage to a secondary winding 10s of the transformer. This pulse voltage is rectified and smoothed by a diode 11 and a secondary smoothing capacitor 12 and is output as a direct-current voltage Vout1. The voltage Vout1 is supplied to an error amplifier circuit including resistors 15, 16, and 13, a shunt regulator 14, and a photocoupler 8s. The FET 9 thus performs PWM switching based on the error information of the voltage Vout1. With this configuration, the voltage Vout1 is made constant.
Next, the DC/DC converter 102 serving as a second converter will be described. The output voltage Vout1 of the AC/DC converter 100 is supplied, as an input voltage to the DC/DC converter 102, to an FET 30 serving as a switching element. To a gate terminal of the FET 30, a push-pull circuit including transistors 27 and 28 is connected via a resistor 29. This push-pull circuit functions as a drive unit for driving the FET 30. The push-pull circuit is used to enhance the speed of switching by enhancing the speed of charging and discharging of a gate input charge Qg of the FET 30. To the push-pull circuit, a PWM circuit serving as a pulse signal generating unit and including resistors 25 and 24, a transistor 26, comparators 23, 22, a triangular wave generator 20, and a constant voltage source 21 is connected. This PWM circuit outputs a pulse signal (hereinafter, also referred to as a PWM signal) for switching the FET 30. The FET 30 performs PWM switching based on a PWM signal that is output based on error information of the output voltage fed back to the comparator 22. With this structure, the pulse voltage is supplied to an inductor 31 and a diode 32. This pulse voltage is commutated by the inductor 31, the diode 32, and an electrolytic capacitor 33 and output as an output voltage Vout2. The output voltage Vout2 is divided by resistors 34 and 35 and supplied to the comparator 22. Thus, as described above, the FET 30 performs PWM switching based on the error information of the voltage Vout2. With this configuration, the voltage Vout2 is made constant. Here, the above-described PWM signal is a pulse width modulation signal and “PWM switching” refers to a switching operation of the FET according to a time width of the pulse width modulation signal.
In FIG. 9, a power saving signal (hereinafter, referred to as a /PSAVE signal) is supplied from the control unit 103 to the AC/DC converter 100. When shifting to the power saving mode is instructed by a /PSAVE signal from the control unit 103, the output voltage of the AC/DC converter 100 is reduced. Then, the operating state is switched to the power saving mode to realize reduction of power consumption while the FET 30 of the DC/DC converter 102 is kept in an on state constantly.
Here, when a general element is used as the FET 30, an ON threshold voltage to keep the general FET constantly in an on state is often equal to or greater than 2.5 V, for example. When such an element is used, for example, if a gate terminal voltage of the FET is lower than 2.5 V in the power saving mode, a case where the FET cannot certainly be turned on may occur in the switching power supply illustrated in FIG. 9. Regardless of FETs, when a power supply circuit includes general elements or parts, the ON threshold voltage becomes lower than 2.5 V. To solve this issue, an element that has a low ON threshold voltage can be used. However, an FET having a low ON threshold voltage may be expensive since a finer semiconductor process is to be used to manufacture such an FET to improve the sensitivity of its gate terminal.
Further, when an expensive FET having a low ON threshold voltage is used, a withstand voltage between its drain and source tends to be low since such an FET is manufactured in a fine process. However, in the above-described switching power supply, the output voltage of the AC/DC converter in a normal mode becomes high so that a high drain-source withstand voltage of an FET is to be used. Thus, such an expensive FET having a low ON threshold voltage does not have a sufficient drain-source withstand voltage.